1. Field of the Invention
This invention relates to furnaces for heating glass articles, in particular glass sheets, of the type wherein the glass sheets are subjected to blasts of hot gas to heat and support the glass on a layer of hot gas. Such furnaces are typically employed to heat the glass in preparation for tempering, bending, or other thermal treatment such as coating at an elevated temperature. Such furnaces incorporate apparatus known as a gas hearth for heating and supporting glass sheets.
Typically, in a gas hearth furnace, gas that is used to support and heat glass sheets is recirculated and reheated to a desired temperature within a gas recirculation system by a burner which uses natural gas or oil for energy. Since natural gas and oil are expected to become scarce in many area of the country in the near future, many of the presently existing gas hearth furnaces will probably have to use electricity as their primary source of energy. If electricity is used, the success in converting existing furnaces to electric energy, or in designing and building new furnaces that use electrical energy instead of fossil fuel energy such as oil or natural gas or coal, will depend upon electric heater designs that are efficient, inexpensive, durable, and easy to maintain.
Unless such conversion can be made, a considerable capital investment in gas hearth furnaces will have to be depreciated entirely. Under such circumstances, the economic burden of such a loss would be considerable.
2. Description of the Prior Art
In the past, electrical energy has been used to supplement energy from fossil fuels in typical gas hearth furnaces of the prior art, overhead electrical heaters have been used in combination with gas hearth support beds and gas hearth furnaces in U.S. Pat. No. 3,223,501 to James C. Fredley and George E. Sleighter, for example. U.S. Pat. No. 3,526,491 to Sharrock suggests incorporating heating elements in a gas hearth bed, particularly near the sides of a path of travel of glass sheets, in order to assist glass sheets to bend. In addition, U.S. Pat. No. 4,059,426 to Eugene W. Starr and U.S. Pat. No. 4,059,427 to Eugene W. Starr and George W. Misson use electrical energy to heat a tube for supplying hot furnace gases preferably in conjunction with an air flow amplifier utilizing the Coanda effect. Such electrical heating as provided in the past has not been sufficient to eliminate the use of other fossil fuels as a heat source for gas hearth furnaces, even though they do provide some improvement over the efficiency of earlier gas hearth furnaces.
A typical gas hearth furnace has a ceramic bed provided with one or more ceramic blocks that include an upper surface over which fluid or hot gas may flow to maintain the treated glass in spaced relationship thereto. A plurality of supply passages are provided within each ceramic block for supplying hot gas from a pressurized gas chamber to the upper surface of the gas hearth block. Other exhaust passages are provided within the gas hearth block for removing excess gas from the space between the upper surface of the block and the glass into an exhaust region where the exhausted gas is recirculated. As stated previously, the recirculation has included a reheating of the circulated hot gases in order to conserve energy. In addition, a blower has been utilized in the recirculating system to further improve the efficient operation of the energy supply.